Rear Oxygen Sensor Heater Test -P0141 (1996-2003 3.8L GM)

TEST 2: Verifying The Heater Element Is Getting Ground

Now that you have confirmed that the rear oxygen sensor's heater is getting power, the next step is to check that it's getting Ground.

Depending on the year of your specific vehicle, the wire that feeds the rear O2 sensor's heater with Ground is a black (BLK) wire or a black with white stripe (BLK/WHT) wire.

IMPORTANT: The pinout in the illustration above is of the connector on the oxygen sensor itself. To check for power, you need to test the BLK wire of the engine wiring harness sensor connector.

These are the test steps:

1. 1

   Locate the BLK wire of the O2 sensor's engine wiring harness connector.
   
   NOTE: Remember, you'll test the wire that's on the engine wiring harness connector side and NOT on the O2 sensor itself.

2. 2

   Place your multimeter in Volts DC mode and connect the red multimeter test lead to battery (+).
   
   With the black multimeter test lead, probe the BLK wire of the O2 sensor's harness connector.

3. 3

   With the Key On, engine Off, this wire should have 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: The rear O2 sensor's heater is being fed Ground. This confirms the rear O2 sensor's heater is being fed with Ground.

So far you've confirmed that the downstream O2 sensor's heater element is getting both power and Ground. The next step is to check the heater element's resistance with your multimeter. For this test, go to: TEST 3: Testing The Heater Element's Resistance.

CASE 2: The multimeter confirms that the BLK wire IS NOT feeding Ground to the rear O2 sensor's heater. Re-check all of your connections and make sure you're testing the correct terminal.

If your multimeter still doesn't register the 10 to 12 Volts DC, then the most likely cause of this missing Ground is an 'open' in the BLK wire between the O2 sensor's harness connector and chassis Ground.

TEST 3: Testing The Heater Element's Resistance

So far, your rear O2 sensor heater tests have found that:

• A P0141 trouble code is stored in the PCM's memory (and lighting up the check engine light).
• 10 to 12 Volts DC are available to the rear O2 sensor's heater (TEST 1).
• Ground is available to the rear O2 sensor's heater (TEST 2).

For our last test, we're gonna' verify that the rear O2 sensor's heater resistance is within factory specification.

If the resistance is not within specification, then we now know the O2 sensor is bad and the cause of the P0141: HO2S Heater Circuit Sensor 2 trouble code.

NOTE: Just a reminder that the downstream oxygen sensor has to be completely cold before proceeding with this test since the manual calls for the O2 sensor to be at room temperature for the resistance test.

OK, this is what you need to do:

1. 1

   Locate the O2 sensor terminals C and D of the O2 sensor connector itself (not the engine wiring harness O2 connector).

2. 2

   With your multimeter in Ohms mode, probe terminals C and D of the O2 sensor itself.

3. 3

   If all is OK, you should see about 3 to 10 Ωs on your multimeter (you'll typically see about 5 Ohms).
   
   If the heater element is fried, your multimeter will show an open (usually indicated by the letters OL) or a number over 10 K Ωs.

Let's take a look at your test results:

CASE 1: The rear O2 sensor's heater resistance is within spec.. This test result tells you that rear oxygen sensor's heater is OK.

CASE 2: Your multimeter showed an open circuit (OL). This confirms that the downstream (Bank 2 Sensor 1) O2 sensor's heater element is fried. Replacing the downstream O2 sensor with a new one will solve the P0141 trouble code lighting up the check engine light (CEL).

Here are some more specifics: Since you have:

• Confirmed that the rear O2 sensor's heater element is getting power (TEST 1).
•      -AND-
• Confirmed that the rear O2 sensor's heater element is getting Ground (TEST 2).
•      -AND-
• In this test, you have confirmed that the heater element's resistance is out of specification.

Taking all of the above into account you can correctly conclude that the downstream O2 sensor needs to be replaced with a new one.

More 3.8L GM Diagnostic Tutorials

You can find a complete list of 3.8L GM tutorials in the following 2 indexes:

• GM 3.8L Index Of Articles.
• GM 3.8L Index Of Articles (at troubleshootmyvehicle.com).

Here's a small sample of the tutorials you'll find in the indexes:

• GM 3.8L Ignition Control Module And Crank (3X, 18X) Sensor Test.
• How To Test The Ignition Coil Pack -Misfire Troubleshooting Tests (GM 3.8L).
• How To Test The GM 3.8L Throttle Position Sensor (TPS).
• How To Test The 3.8L GM Cam Sensor (P0341) (at: troubleshootmyvehicle.com).